The invention pertains to a method and to an apparatus for drive slip control wherein the engine torque is reduced when the actual slip exceeds a predetermined desired slip which is adjustable as a function of a measured operating variable.
In acceleration processes on roads with low coefficients of friction, the driver often applies too much gas, which causes the drive wheels to spin. In these cases, the drive slip control (ASR) adjusts the drive slip to a value calculated in the control unit. This slip determines the traction and the stability (steering stability) of the vehicle.
It is true for many road surfaces that high traction also requires a high desired slip, whereas good vehicle stability requires a low desired slip. The process of arriving at the desired slip value is therefore a compromise between the maximum achievable traction and the minimum acceptable stability.
To improve this situation, the desired slip is adjusted automatically to the velocity of the vehicle and to travel around curves. During straight-ahead travel, the desired slip is usually calculated by adding a velocity-independent value to a velocity-dependent value. In the normal case, these values are determined by driving tests in such a way that the relative desired slip (desired slip relative to the velocity of the vehicle) of the drive wheels decreases with increasing velocity, so that better vehicle stability is achieved at higher velocities. At lower velocities, the relative desired slip is high; it therefore gives greater traction in many cases.
An approach such as this is known from, for example, EP-A 166 178.
To guarantee the steering stability of the vehicle at low velocities even when traveling around a curve, the desired slip is usually reduced as a function of the velocity and a radius of curvature estimated from the wheel speeds. This approach is known from, for example, GB 2,188,996.
During fast or slow driving on mountain roads, the way in which the desired slip is calculated leads in many cases to a reduction in the engine torque by the drive slip control. As a result, the engine torque is no longer sufficient, and the vehicle slows down. In extreme cases, the vehicle can even come to a complete standstill.
U.S. Pat. No. 5,313,391 discloses a drive slip control system, in which the desired slip is calculated as a function of the vehicle's acceleration to achieve an improvement in the traction. In addition, the desired slip is corrected as a function of the square of the vehicle's velocity to equalize the tire tolerances, and the driver's influence on the determination of the slip threshold and thus on the coordination between traction and stability is also taken into account. This is accomplished by weighting the vehicle's acceleration and the square of the vehicle's velocity with the position of the gas pedal or the position of the throttle valve. Nevertheless, as the acceleration decreases and the velocity of the vehicle drops, the driver's influence decreases.